Last Updated on October 31, 2022 by Stephanie
The reason that rosemary leaves change color is due to fungal diseases that are caused by moist soil around the roots or by high humidity. Botrytis, root rot, black spot, and other fungal pathogens may cause rosemary to change color to black.
The rosemary plant originates from the Mediterranean which is where it thrives in soils that are sandy or stony which drain quickly and flourish in full sun , with no rain or irrigation.
The conditions that make your rosemary black are usually due to over-watering, slow draining soils, heavy humidity, heavy rainfall and excessive watering. This can create conditions for the fungal pathogens which cause rosemary to turn black and flourish.
Rosemary likes its roots to dry out in between watering sessions and dry conditions. To prevent and treat rosemary stems and leaves becoming black, it is essential to mimic certain conditions that are typical of the natural environment of rosemary.
Read on to find out the reason your rosemary is turning dark and the best methods for stopping the problem, and ways to fix the issue…
Table of Contents
Over Watering Promotes Fungus Causing Rosemary to Turn Black
The pathogens of fungal diseases that cause rosemary disease are aggravated by moist soils.
The native rosemary is found in Mediterranean areas typically in stony or sandy soils that are found on hillsides. Its climate in the Mediterranean region is characterized by a lot of sunshine and dry, arid conditions and frequent rain.
The scent and taste of rosemary are at their peak during the seasons of frequent rain, scorching sunlight and soils that are comparatively poor in nutrients.
So, rosemary doesnt require a lot of water to flourish. The majority of gardeners water rosemary, which can cause environment for fungal illnesses which can lead to root rot, or even change the leaves of rosemary into black.
In the majority of climates, established rosemary doesnt require additional water, except in climates that have substantial rainfall
- Reduce the amount of water you are able to give once each two weeks but only do this during the summer months, if there isnt been any significant rainfall, if the rosemary is in the soil in the garden.
- Pots for rosemary and containers requires watering every two weeks, even if there is a rainfall because pots dry out faster than soil for gardens.
- Dont water rosemary during winter as it will get all the water it needs from the surrounding environment, if it is outside. Also, watering at this time of year can make the rosemary more susceptible to fungal pathogens that cause leaves the stems, leaves and even the stems to become black.
In reducing the amount of water, the roots will get the chance of drying out which can help fight the fungal diseases as well as root rot.
Slow Draining Soils
The rosemary plant has been adapted to grow in well-draining and sandy soils, typically on the hillsides of Mediterranean countries like France and Italy The roots are used to well-drained, dry soils.
When you plant rosemary in the garden, a typical error is planting it into moist soils that retain moisture, such as the clay or rich compost soils that drain slowly.
The moisture around the roots caused by slow draining soils is ideal conditions for fungal pathogens to flourish that causes rosemary to darken.
What should I do…
To grow rosemary effectively and avoid or treat it for the black leaves, it is crucial to mimic the well drainage soil conditions found in the Mediterranean.
When you plant rosemary, whether in a pot or a garden boarder make sure to amend the soil with a grit or sand mixture prior to planting.
This will increase the drainage dramatically by increasing the porosity of soil, and also balance the soils properties so that it doesnt hold water which could harm the roots and create the conditions that favor fungus that transforms rosemary black.
If you are digging a hole to plant your rosemary, or making containers and pots to be used, you should add around 20 percent sand or grit the area , and then add 80percent multi-purpose compost.
(Read my article on the the best soil to plant rosemary in pots).
In areas with high rainfall, I suggest including a higher percentage of either sand or gravel in order to make sure that soil can drain quickly , so that the roots remain dry and stay healthy.
If your garden soil is sloppy or contains a greater amount of clay, I would suggest planting or transplanting rosemary into containers or pots.
Containers and pots are an excellent option to plant rosemary since containers have better drainage, and also you manage the soils profile instead of making amends to soil thats not the best choice.
The growing of rosemary in pots allows you to relocate the pot to a shelter during an occurrence of heavy rainfall, if the rosemary is showing indications of becoming black.
Check out this YouTube video to learn how to transplant rosemary , so you can transplant the plant without risk from a slowly draining soil.
Over Head Watering Spreads Fungal Spores
Always sprinkle water on rosemary near the bottom of the plant, not over the foliage. The foliage that is constantly damp can create humid conditions that promote mildew (mildew generally grey) as well as spots on the leaves and leaf spot, both of which could cause the foliage to turn back.
The spores of fungus can be transported by water, so it is best to water the roots of the plant, rather than sprinkle water on the leaves.
Rosemary is able to tolerate wet foliage following a rain when it is located in a hot climate under full sun, in which it is more likely that the amount of water vapor is higher.
In warmer climates, the water may remain on the leaves for a longer period, (particularly on overcast or colder days) which increases the possibility of fungus.
The rosemary should be watered at the base to ensure that the water gets to the roots, where it is needed instead of sitting on the leaves, creating an ideal environment for fungus, which can make the foliage to black.
(Read my article on on how you can water your rosemary to determine the best time to water the garden).
Humidity encourages The Spread of Fungus, which can cause Spots that are black. Spot
As with many Mediterreanean herbaceous plants, rosemary likes to thrive in full sun in dry areas that has low levels of humidity.
If you plant rosemary too close to each other or in a crowded area and in an area that has very little air circulation (in the corners of a patio, for instance) is when a microclimate could develop, which can increase the humidity in the space.
If the climate is too humid and there is no airflow, this could increase the chance of
- Leaf spot
Humidity may also contribute towards an increased risk of the possibility of root rot, and can cause the leaves of rosemary, and even the stems, to black.
Follow these steps to minimize the chance of the rosemary becoming black because of the humidity…
- If you plant in the garden or in pots, place each rosemary about 3 feet from each other. This will ensure that each plant is given enough room for its roots to develop, and also let air move through the leaves, which will keeps it dry and lowers the chance of fungus.
- Find your rosemary within an open part of your garden, perhaps with a breeze. The rosemary plant naturally thrives on the coasts and hills therefore it is able to withstand a lot of winds.
- If your area is humid, then gardening in containers and pots are the ideal choice. Containers and pots have good drainage, and they can improve airflow since they are elevated off the ground, in contrast to garden boards.
How to Treat Fungus and Black Spot on Rosemary Leaves
The rosemary plant is usually a sturdy plant that is immune to diseases, so even when the issue is a mildew or black spots, the plant is usually able to be cured with proper treatment.
- Pick a day that is dry when it is possible to take care of your rosemary. The fungal spores that grow on leaves, disperse and multiply because of water, therefore, do not try this following a rainy day.
- Make use of sterile pruners to cut off the stems or leaves which are black. Clean the blades using an soaked cloth in disinfectant after each cut to stop the spread of the fungus (the spores could be transferred onto cutting blades).
- Burn or eliminate the black foliage that is affected than putting it in the compost pile as the fungus may remain dormant and grow throughout the garden if you distribute the compost.
Follow these steps to determine if the area of rosemary that has good aeration. The rosemary will grow back.
Apply a fungicide organic to the rosemary to make sure that the fungal spores do not spread throughout the rosemary.
Heres a fantastic YouTube video on mixing the fungicide of your choice organically:
Pots and Containers
If your rosemary is turning black inside the container or pot, make sure that the pot has drainage holes. Also, ensure that you dont collect any excess liquid in the drip tray since this results in the soil becoming saturated with water and causes root rot, which can result in the rose to change color to black.
Sprinkle some grit on at the base of your pot to ensure the drainage holes arent blocked by soil that has been compacted.
Select a pot that measures 16 inches wide to make sure that the pot is able to hold to hold enough soil so that the roots remain protected in winter and that the rosemary doesnt get stuck in the pot, which could cause leaves of the leaves of rosemary brown as well as result in rosemary to fade away.
How to Treat Rosemary That is Turning Black Due to Root Rot
If your rosemary is slowly becoming black, the only method to save it is to cut off the affected roots and growth in order to stop its spread of the disease, but rosemary is often killed after root rot has occurred or Botrytis blight has afflicted the plant.
- The first step is to take the rosemary from the pot, or remove it remove it from the ground using a fork , and then examine the roots.
- Take out roots that appear dark and slimy, as opposed to lighter, healthier color roots.
- Make use of a disinfectant-based cloth to clean the blades between each cut to stop the spreading of the fungal infection.
- Cut off any affected leaves and stems, then burn them , or take the plants in your garden. Get rid of the soil elsewhere because it could contain fungal spores which could then spread to other plants. Treat the soil surrounding it with the chemical fungicide.
- Plant the roses in a fresh pot using soil which has been amended with sand or grit in order to improve the drainage.
- Pots are more efficient in drainage, which can help fight the fungal infection.
- Set the pot 3 feet from the other pots, and place it the pot in full sunlight.
If you cut off the affected parts of the plant, you provide the plant with a shot to recover, though its often more convenient to buy a fresh rosemary plant and then throw away the soil in the pot and plant because it could contain fungus.
(Read my article on selecting the most suitable containers for the rosemary).